Generally a material, such as a photothermographic material is produced by coating a support with a light sensitive silver halide composition, such as a silver halide-organic silver salt-polymer emulsion, and (or) with a coating solution for a non-light sensitive layer, (either to be referred to hereinafter generically as a "coating composition"), and drying the coating. The general practice is to incorporate various additives in the coating composition during its preparation. Some types of additives, however, may react with the polymer binder or other chemicals in the coating composition, and will adversely affect the properties of the coating composition or of the coatings. These include the photographic characteristics of the imaging element, the Theological behavior of the coating solution and the physical properties of the coated layer, such as reticulation, adhesion, melting point, abrasion resistance, wet-ability, anti-static properties etc. When such additives are used, the coating composition must be applied immediately after the preparation of the coating composition in order to avoid adverse effects. This imposes a restriction on the use of fast-acting additives. Furthermore, according to this practice, an additive whose distribution should desirably be controlled in a particular area, for example a surface modifier such as anti-static agents, lubricants, etc. which should desirably be distributed and concentrated near the surface of the coating, are difficult to incorporate in such a manner as to achieve the desired distribution.
For many applications, the coating additives are incorporated directly into the coating solution and then coated onto various supports. For photographic or (photo)thermographic materials, "hardeners" or crosslinking materials may be used as additives in the coating solution to improve the properties of the dried coating composition. Many of these hardeners are fast acting additives and basically react before the solution can be coated. When this occurs, the additives can adversely affect the coating solution (viscosity, wet-ability, etc.) or product properties (adhesion, melting point, abrasion resistance).
In U.S. Pat. No. 4,218,533 to Fuji, a process of by which coating additives (such as hardeners) are applied independently of the coating solution is discussed. The additives are added via atomization due to ultrasonic vibration. The atomization process generates small droplets which are essentially sprayed onto the wet coated surface. Once the droplets come in contact with the wet coated surface, they begin to react.
The benefits of atomization are: 1) it removes defects associated with the general methodology of incorporating fast acting additives, 2) it increases batch life by keeping additives out of the coating solution prior to application and, 3) it provides capability of incorporating additives in a desired localized or limited area.
The limitations of the prior art method are primarily that the droplets will cause surface disruptions (thus defects) when sprayed directly onto the surface of the wet (undried) coating surface and uniform distribution of the spray across the web. In many instances, the additive must be added to the wet coating surface in order for the additive to be effective (i.e. it needs to react with the wet solution). The patent also describes applying the additive after the surface is dried to eliminate the surface disruptions caused by the spraying, but this may limit the reactivity of the additive.
Agfa U.S. Pat. No. 5,443,640 describes the use of applying atomized droplets to form a thin antistatic layer on a dried surface of a coating. It also describes a process for controlling the droplet flow which requires special air flow handling stations. This patent does not teach the use of reactive materials in the process.
It is known in the art, that many different materials can be applied to coatings and supports using vapor deposition such as in U.S. Pat. No. 5,236,739 (H.-H. Chou, et. al), U.S. Pat. No. 4,094,269 (Y. P. Malinovski, et. al) and U.S. Pat. No. 4,954,371 (A. Yialixis). Although reactive materials may be used, the methods described in these patents require vacuum, high temperature or both which is not desirable for a continuous coating process which uses organic solvents.
Fast reacting materials may also be incorporated into the composition by applying them in a multilayer coating format, such as a simultaneous slide layer. This requires the appropriate selection of coating solvents for all the layers to prevent phase separation. Although this method keeps the reactive additive away from the main coating elements, it typically generates increased waste due to defects such as lines, streaks and width-wide crosslines in the coating associated with the additional coating layer.
It would be considered highly advantages to be able to coat all kinds of additives uniformly to all kinds of coatings and to be able to coat either wet or dry coatings.